- A backbone: Fishes are vertebrates, which means that they have a backbone. This is a long, flexible bone that runs along the back of the animal and provides support for the body.
- Paired fins: Most fishes have paired fins, which are located on the sides of the body. These fins help the fish to swim and maneuver.
- Scales: Most fishes have scales, which are small, hard plates that cover the body. These scales help to protect the fish from injury and infection.
- Gills: Fishes breathe through gills, which are located on the sides of the head. Gills are made up of thin filaments that are covered in blood vessels. As water passes over the gills, the oxygen in the water is absorbed into the blood vessels.
- A swim bladder: Many fishes have a swim bladder, which is a gas-filled sac located inside the body. The swim bladder helps the fish to control its buoyancy, which is the ability to float in water.
Not all fishes have all of these characteristics. For example, some fishes, such as hagfish and lampreys, do not have scales. And some fishes, such as lungfish, have lungs in addition to gills.